1. Field of the Invention
The present invention relates to an actuator which composes a micro-machine or MEMS (Micro-Electro-Mechanical Systems), which is used as elements having a moving part, such as a switch and a variable capacitor.
2. Description of the Related Art
Actuators compose main sections of elements having moving parts such as switches and variable capacities (for example, see U.S. Pat. No. 6,483,395).
Actuators include electrostatic actuators. Electrostatic actuators adopt a bridge structure (both ends of a moving part are supported), and a cantilever structure (only one end of a moving part is supported).
The bridge structure has an advantage in that warpage of the moving part due to a residual stress is reduced, but at the same time, this structure has a disadvantage in that temperature dependence of a pull-in voltage and a pull-out voltage becomes great.
This temperature dependence is caused because thermal expansion coefficients of a substrate and a moving part (bridge part) are different and a spring constant K of the moving part has temperature dependence. However, it is difficult to eliminate this.
When the problem of this state is left unsolved, however, a reduction in a pull-out voltage due to the temperature dependence produces a phenomenon in which the moving part cannot be separated from an electrode (stiction).
The pull-in voltage and the pull-out voltage are fluctuated between electrostatic actuators in one chip or different chips by so-called process tolerance.
It is, therefore, desired to develop a technique which prevents deterioration in reliability of an electrostatic actuator due to the fluctuation in the pull-in voltage and the pull-out voltage.